1. Field of the Invention
The present invention relates to an electroless gold plating solution and an electroless gold plating method. In particular, the present invention relates to a reduction type electroless gold plating technique capable of directly subjecting a plated coating film made of an underlying metal such as copper, nickel, or palladium to plate processing.
2. Description of the Related Art
In recent years, development of electronic components and semiconductor parts has advanced. An advanced mounting technique for realizing a small-sized semiconductor package having a large capacity has been required. Therefore, for example, a plating technique has been known, which forms a circuit pattern on a substrate using a metal such as copper having low electrical resistance when manufacturing the semiconductor package, and further performs nickel plating, palladium plating, and gold plating to form a joined part.
A nickel plated coating film is used as a barrier film for preventing erosion of a copper circuit caused by solder. A palladium-plated coating film is used as a barrier film for preventing diffusion of the nickel plated coating film to a gold plated coating film. Since the gold plated coating film has low electrical resistance and good solder wettability, the gold plated coating film is applied to the final finish. Therefore, a joined part having excellent joining properties such as soldering or wire bonding, can be formed with a plated coating film made of an underlying metal such as nickel or palladium and the gold plated coating film.
As the above-mentioned plating technique, there has been known a method for subjecting the underlying metal such as palladium to immersion gold plate processing to secure adhesion between the plated coating film and the underlying metal. However, since the immersion gold plate processing stops the reaction when the underlying metal is wholly substituted, the immersion gold plate processing limits a film thickness capable of being formed. On the other hand, formation of a thick gold plated coating film may be required for a portion joined with wire bonding. In order to form the thick gold plated coating film, gold plate processing is performed, which has two steps of subjecting the underlying metal to immersion gold plate processing to secure adhesion and then further subjecting the underlying metal to reduction type electroless gold plating. For example, Patent Document 1 discloses an electroless gold plating solution made of a gold ion, a complexing agent, a thiourea compound, and a phenyl compound as a reduction type electroless gold plating solution used after the immersion gold plate processing.
A plate processing operation itself is complicated by subjecting the underlying metal such as palladium to immersion gold plate processing and then subjecting the underlying metal to reduction type electroless gold plate processing. The immersion gold plate processing deposits gold using the difference between oxidation-reduction potentials of the plated coating film and the underlying metal. The immersion gold plate processing may partially form severe corrosion of the underlying metal. Another problem pointed out is that such a defect reduces joining properties. For example, Patent Documents 2 and 3 can realize gold plate processing which suppresses the corrosion of the underlying metal. These electroless gold plating baths can suppress the corrosion of the underlying metal. However, the unstable electroless gold plating baths and unfavorable appearance of the gold plating thereof are pointed out.
An electroless gold plating solution containing at least one kind of compound selected from the group consisting of formaldehyde bisulfites, rongalit, and hydrazines as a reducing agent (see Patent Document 4), and an electroless gold plating solution containing a water-soluble gold salt, a complexing agent, and an aldehyde compound having a predetermined structure (see Patent Document 5) have been proposed as an electroless gold plating solution improving the above-mentioned conventional technique. These electroless gold plating solutions have a desired deposition rate, and provide a good appearance. The underlying metal such as nickel or palladium can be also directly subjected to gold plate processing. However, since the solutions include toxic formaldehyde, the solutions are not preferable for a plate processing environment.